It is well known to form an image on an illuminated surface of a body by absorption or blocking of energy of an illuminating beam. For example in an overhead projector, an over-head transparent absorbs or blocks part of the light beam of the projector whereby a large image of an overhead is formed on a screen. However, this results in a loss of light intensity as part of the emitted light from an image forming system is reflected or absorbed.
To avoid loss of energy causing, e.g. loss of light intensity of the synthesized intensity pattern, power dissipation generating heat in components of the system, etc., methods and systems have been developed wherein the phase of a light beam is modulated instead of the amplitude or intensity of the light beam, as modulation of the phase of the light beam do not lead to loss of energy. The phase modulation is followed by a conversion of the phase modulation into an amplitude or intensity modulation.
A diffractive optical element, such as a holographic optical element, may be used to generate a phase modulation. Then, the resulting intensity modulation at each point of a picture formed by conversion of the phase modulation into intensity modulation will depend upon the phase modulation values at each point of the diffractive optical element as the light intensity at each point of the picture is formed by a coherent superposition of light received from the entire surface of the diffractive optical element. Diffractive optical elements are rather complex to design for synthesis of a prescribed intensity pattern.
Imaging methods and systems may also be used in connection with phase modulation. These methods and systems are characterized by the fact that the intensity of a point of a picture formed by conversion of phase modulation into intensity modulation will depend upon the phase modulation value of one point of the phase modulator only as this point is imaged onto the picture point in question by the imaging system. This one-to-one relationship makes the design of phase modulators in these systems simple. Methods and systems of this kind are named phase contrast imaging methods and systems.
Phase contrast imaging methods were originally developed within the field of microscopy. Many objects of interest in microscopy are largely transparent, thus absorbing little or no light. When light passes through such an object, the predominant effect is the generation of a spatially varying phase shift which can not be seen by a human as the eye of a human responds to light intensity and colour and does not respond to the phase of light.
In 1935, Fritz Zernik proposed a phase contrast technique which rests on spatial-filtering principles and has the advantage that the observed intensity is linearly related to the phase shift introduced by the object.
Suppose that a transparent object with amplitude transmittance EQU t(x,y)=exp[j.phi.(x,y)] (1)
is coherently illuminated in an image-forming system. For simplicity, a magnification of unity is assumed and the finite extent of the exit and entrance pupils of the system is neglected. Further, a necessary condition to achieve linearity between phase shift and intensity is that the phase shift .phi. be less than 1 radian, in which case the amplitude transmittance can be approximated by EQU t(x,y)=1+j.phi.(x,y) (2)
The terms of order .phi..sup.2 and higher are neglected in this approximation. It is seen that the first term of (2) leads to a strong wave component that passes through the sample without change, while the second term generates weaker diffracted light that is deflected away from the axis of the system.
The image produced by a conventional microscope can be written EQU I.apprxeq..vertline.1+j.phi..vertline..sup.2 .apprxeq.1 (3)
where the term .phi..sup.2 has been approximated by zero. It is seen that the diffracted light is not observable because it is in phase quadrature with the strong background. As Zernik recognized that the background is brought to a focus on-axis in the focal plane while the diffracted light--containing higher spatial frequencies--is spread away from the focal point, he proposed that a phase-changing plate be inserted in the focal plane to modify the phase relation between focused and diffracted light.
The phase-changing plate can consist of a glass substrate on which a small transparent dielectric dot has been coated. The dot is placed at the center of the focal plane and has a thickness and index of refraction such that it retards the phase of the focused light by either .pi./2 radians or 3.pi./2 radians relative to the phase retardation of the diffracted light. In the former case the intensity in the image plane becomes EQU I=.vertline.exp[j(.pi./2)+j.phi..vertline..sup.2 =.vertline.j(1+.phi.).vertline..sup.2 .apprxeq.1+2.phi. (4)
while in the latter case EQU I=.vertline.exp[j(3.pi./2)+j.phi..vertline..sup.2 =.vertline.j(-1+.phi.).vertline..sup.2 .apprxeq.1-2.phi. (5)
Thus, the image intensity has become linearly related to the phase shift .phi.. When the phase of the background is retarded by .pi./2, the result is known as positive phase contrast, while a 3.pi./2 retardation is said to yield negative phase contrast.
It is seen that the method described above leads to a phase contrast imaging method that provides a small phase signal that is superimposed on a large DC-component. This leads to an important disadvantage of the method because, typically, it will be necessary to attenuate the DC-component to enhance the information contained in the phase modulated signal. However, the attenuation of the DC-component leads to loss of energy. This kind of filtering is usually denoted Dark Field Filtering.
It is another disadvantage of the phase contrast imaging method described above that it is based on the assumption that the phase shift .phi. is less than 1 radian which is very often not fulfilled in practical real-life applications. However, the theory is still applied to such applications, disregarding the fact that the basic assumption is not fulfilled, and this leads to non-optimized technical solutions.
In EP 0 657 760 a phase contrast imaging system is disclosed in which an image simulation and projection system is based on the Texas Instrument flexure beam digital mirror device (DMD). The flexure beam DMD is used for analog phase modulation of reflected light and the phase modulation is converted to amplitude modulation utilizing a phase contrast imaging method. The flexure beam DMD provides a flicker-free modulated wave and accordingly, optical image sensor synchronization is not needed. The system disclosed operates according to the Zernike method and, thus, includes the corresponding disadvantages described above.
Another example of a phase contrast imaging system is disclosed in GB 2 199 716, wherein an optical guide-beam projector for a missile guidance system is disclosed that provides a spatially intensity modulated guide-beam. A spatial phase modulator is used to generate the guide-beam. The phase encoding of the spatial phase modulator constitutes a periodic square-wave modulation (50% duty cycle) of two phase values 0 and .pi./2. The phase modulation is converted into an amplitude modulation by Fourier transforming lenses and a phase plate providing a phase shift of the background signal by .pi./2. A method for synthesizing the specific intensity pattern of the optical guide-beam based on phase contrast imaging is not disclosed in this document.
A similar example of a phase contrast imaging system is disclosed in "Array illuminator based on phase contrast", Applied Optics Vol. 27, No. 14, pp. 2915-2921 (1988). A method is disclosed of converting a wide beam of uniform intensity into an array of bright spots without losses. The input spatial phase mask constitutes a periodic array of phase dots with the phase value .pi., the remaining area of the phase mask having the phase value 0. The phase modulation is converted into an amplitude modulation by Fourier transforming lenses and a phase plate providing a phase shift of the background signal by .pi.. The method is limited to the implementation of periodic array configurations with the binary phase values 0 and .pi..
It is well-known to use so-called "radiation focusators", i.e. computer generated holographic optical elements, for spatial phase modulation of a light beam, e.g as disclosed in Special Issue on Computer Optics in the USSR, Optics and Lasers in Engineering, Vol. 15, no. 5 1991. However, such elements are complicated to synthesize. Typically, they are synthesized in such a way that the desired image is formed in the Fresnel region or the Frauenhofer region. Thus, the intensity of a resolution element in the generated image is a function of several, typically all, phase values of the resolution elements of the holographic optical element. Obviously, this complicates the design of a general purpose holographic optical element and advanced, very time consuming algorithms have to be applied. Further, the complicated design of the holographic optical elements renders it almost impossible to implement dynamically changeable spatial phase modulators with such elements.
It is a further disadvantage of holographic optical elements that a carrier frequency is needed to separate diffracted light from non-diffracted light resulting in an off-axis system geometry and a need for a diffractive medium that can support these high frequency terms.